1. Field of the Invention
The present invention relates to an LCD display unit capable of preventing turbulence of images, generated when the image on the LCD display is switched or the LCD power supply is turned off, and more specifically to an LCD display unit that prevents the turbulence of images in the reflex LCD.
2. Related Art
In replacement of the cathode-ray tube that has been used in various types of information display units, the Liquid Crystal Display (hereunder, referred to as xe2x80x9cLCDxe2x80x9d) has widely been employed, because it has advantages of requiring less space for installation and less power consumption. The LCD has various types. From the standpoint of structure, there is the direct-view type in which a user views the display itself directly, and the projection type in which a user views images projected on a screen. The widely used direct-view type LCD includes the transmission type LCD having a backlight, and the reflex type LCD utilizing the reflected light of natural light and/or a room lamp light. Since it is impossible to present visible images without a surrounding light source, the reflex type LCD includes a type provided with a backlight, so that backlighting can be used as needed.
Because the liquid crystal substance does not emit spontaneously, the transmission type LCD is provided with a backlight on the backside of an array substrate, which irradiates a light thereon. While the light permeates through the array substrate containing the liquid crystal substance and the liquid crystal cells, the liquid crystal produces a torsion effect on the light, whereby the transmission type LCD gives a viewer images obtained by the light transmitting through a polarization plate. In contrast to this, the reflex type LCD makes natural light being irradiated on the surface of the liquid crystal cell substrate reflect on the array substrate and the liquid crystal cell substrate to thereby give a viewer images.
Further, the LCD uses the Nematic liquid crystal substance as represented by the TN type, STN type, DSTN type, etc., which includes the passive matrix LCD as a simple matrix system using passive elements only, not using active elements, and the active matrix LCD inside of which the drive control is carried out by the active elements such as the thin film transistors and diodes represented by the TFT type LCD. In recent years, there have been strong demands for a fine color image with a good response on a wide display such as a personal computer display and a TV display, etc., so that the active matrix LCD has been adopted increasingly.
Such an active matrix LCD will be explained with a representative TFT type LCD as an example, with reference to FIG. 5. Thin film transistors (hereunder, abbreviated as TFT) as the active elements are connected to the intersections of the longitudinal pixel electrodes X and the lateral pixel electrodes Y. One end of each TFT is supplied with a data signal and the other end thereof is connected to a storage capacitor (not illustrated) and the pixels formed by the liquid crystal substance are inserted in parallel with the storage capacitors. Further, the gate electrodes are connected to the lateral pixel electrodes Y, to which address signals are supplied from the outside, and in accordance with the address signals, the data signals are transmitted to the pixels through the TFTs.
The active matrix LCD of the TFT type formed on the foregoing principle is driven and controlled by a drive circuit as shown in FIG. 6, for example. This drive circuit is composed of, to classify broadly, a signal control circuit 32, power supply circuit 34, gradation voltage circuit 33, facing electrode drive circuit 35, address line drive circuit 36 as a gate driver, data line drive circuit 37 as a source driver, and the like, which drives a liquid crystal panel 31 having a structure as shown in FIG. 5. The drive circuit including these control circuits forms a liquid crystal module 30.
In such a drive circuit, as a power supply voltage, clock signals xcfx861, xcfx862, a synchronizing signal, and a data signal are supplied, the signal control circuit 32 supplies the data line drive circuit 37 as the source driver with the data signal, the control signal, and the clock signal xcfx861, and supplies the address line drive circuit 36 as the gate driver with the control signal, clock signal xcfx862. The power supply circuit 34 regulates the power supply supplied from the outside, and supplies a necessary power supply voltage to a driver IC of the data line drive circuit 37 and a driver IC of the address line drive circuit 36. The gradation voltage circuit 33 supplies the data line drive circuit 37 with a gradation voltage used by the data driver for generating an output voltage. Further, the facing electrode drive circuit 35 supplies a common voltage to common electrodes facing the pixel electrodes.
In the TFT type LCD, a gate voltage from the address line drive circuit 36 turns the TFT on/off by row, and during the on interval of the TFT, the output voltage from the data line drive circuit 37 enters a source electrode 45 of a TFT 44 through a data line 43. Through a drain electrode 46, the output voltage is applied across a pixel electrode, which is illustrated by a capacitor 50 of the pixel portion represented by way of the equivalent circuit in the drawing, and a storage capacitor 51 that holds the supplied voltage, which is connected in parallel with the pixel electrode, whereby an image is displayed. Here, the difference between the potential of the pixel electrode and the potential of the facing electrode is the voltage applied to the liquid crystal layer, and this applied voltage presents a liquid crystal image with an appropriate gradation.
After the gate of the TFT is switched off thereafter, the voltage stored across the storage capacitor 51 maintains the displayed image as it is. In order to present the next frame image, the gate voltage is again supplied to the TFT to turn it on, and the reverse voltage to the former is supplied to the pixel and the storage capacitor. Thereby, the charges across the pixel and the charges across the storage capacitor are discharged, and next a specific reverse voltage for presenting an image of a specific gradation is stored across the storage capacitor 51 to present a next frame image. To repeat such operations presents a specific image on the whole display. Here, in FIG. 6, the storage capacitor 51 is connected to a Cs line 42 provided separately from the common electrode 41, however it can be connected to the common electrode 41 without using the Cs line 42.
In the LCD display unit that carries out the foregoing operations, to finish the liquid crystal display from the state of a normal liquid crystal display, all the power supply lines are brought into the off state. Consequently, the power supply line leading to the power supply circuit 34 is switched off, and the data signal line is switched off at the same time. Since the TFT is brought into the off state at that time, the storage capacitor 51 that stores a charge so as to continuously maintain a specific voltage for maintaining the image on each pixel will continue to maintain the state with the charge held. However, the charges are discharged gradually from leakage elements such as an internal resistance of the TFT 44 and the like, which causes uncertain turbulence of the liquid crystal in the pixel portions of the entire display, depending on characteristics such as the discharge characteristics of the elements and the characteristics of the elements that are influenced by the internal and external circuits. Here, in the reflex type LCD, for a period of time after turning off the power, the disturbed images can be seen through the uncertain turbulence of the liquid crystal, which presents blurry, irregular after-images.
With respect to this point, the backlight type LCD that has widely been used is provided with a backlight on the backside of the array substrate that irradiates a light, whereby a user is to view images obtained by the light passing through the liquid crystal. Therefore, turning off the power supply of the LCD will simultaneously turn off the backlight to suppress the transmission of light through the liquid crystal almost completely. Accordingly, there cannot be seen such irregular after-images created in the liquid crystal as the in foregoing phenomenon.
Further, in the backlight type LCD, in order to execute the video muting that temporarily erases images during display, it is not necessary to turn off the power supply, because to turn off the backlight makes almost invisible the state of a normal display image processed in the liquid crystal. Therefore, the video muting in the backlight type LCD has been carried out by turning off the backlight.
On the other hand, the reflex type LCD with recent advancements in research and development, which is being widely employed, makes natural light falling on the surface of the liquid crystal cell substrate reflect on the array substrate and the liquid crystal cell substrate to present an image. Therefore, the phenomenon of the turbulence of the liquid crystal, which is created when the power supply is turned off, is viewed as irregular after-images by reflected light, which detracts from the perception of quality of the display and the equipment that incorporates the display, and gives an unacceptable feeling to a user.
Further, since it is not provided with a backlight as the backlight type LCD has, the reflex type LCD is not able to execute the video muting by the foregoing technique. Accordingly, unless a special measure is taken, a temporal muting of video images requires turningoff the power supply, which gives a user the irregular after-images at each time of the video muting.
Incidentally, in the reflex type LCD, to eliminate the necessity of turning off the power supply at each time of the video muting, it is possible to give an appearance as the video images are muted, in which the video signal input circuit prepares the normal video signal input and the black image signal input so that they can be switched alternately, and when the video mute signal is inputted, the circuit is switched into the black image signal input to display the black image on the LCD display unit.
The present invention has been made in view of the foregoing circumstances, and it is a major object of the invention to provide an LCD display unit that does not produce irregular after-images on the LCD display when the power supply thereof is shut off, even in the reflex type LCD.
According to one aspect of the invention, the LCD display unit displays video images by reflected light for the most part, and includes: a video setting unit that sets either a normal video signal or a specific image display signal as a video signal for image display; a gate voltage control unit that controls a gate voltage of drive elements of pixels; a common electrode control unit that controls common electrodes of the pixels; a source voltage control unit that controls a supply voltage to a data line drive circuit; and a power operation timing controller that controls operation timings of the units when the power supply is turned on/off. In addition to the above construction, the power operation timing controller includes a means that, after the video setting unit sets the specific image display signal when the power supply is turned off, the gate voltage control unit turns off the gate voltage, the common electrode control unit turns the common electrodes off, and then the source voltage control unit turns off the supply voltage to the data line drive circuit.
According to another aspect of the invention, in the LCD display unit, the time at which the common electrodes are turned off is set to the time after an image display by the specific image display signal is maintained for a specific time.
According to another aspect of the invention, in the LCD display unit, a point of time at which the supply voltage to the data line drive circuit is turned off is set to the time at which a common electrode voltage is substantially dissipated.
According to another aspect of the invention, in the LCD display unit, when the power supply is turned on, the power operation timing controller turns on the source voltage control unit, the gate voltage control unit, and the common electrode control unit, and thereafter outputs the video signal to the data line drive circuit.